ARTWORK- A REVIEW OF RESEARCH WORK DONE BY PROFESSOR ARTHUR (Art) E. BERGLES

Satish G. Kandlikar Mechanical Engineering Department

Rochester Institute of Technology Rochester, New York

USA

ABSTRACT

Professor Arthur E. Bergles has made major contributions in a

number of areas in heat transfer. This paper presents a review of

300 papers published by him, and with his students and co-

workers, through December 1996. Professor Bergles research work

can be broadly categorized into seven areas: (i) enhanced heat

transfer, (ii) two-phase flow and heat transfer, (iii) heat transfer to

refrigerants (boiling and condensation), (iv) cooling of electronic

components, (v) laminar internal flow, (vi) review and general

papers, and (vii) history of heat transfer. This research, conducted

over more than 30 years, has produced a wealth of high-quality

experimental data, theoretical models, and their practical

applications. One of the major objectives of this article is to

highlight these contributions and identify their sources, to facilitate

future researchers and designers in developing new theoretical

models and in designing industrial equipment.

1. INTRODUCTION

The research work of Professor Bergles conducted over a span

of over thirty years is a great gift to the heat transfer research

community in academia as well as in industry. Professor Bergles,

through his extensive reports and publications, helped us all in

defining the research needs through his visionary review papers,

then identifying for industry the potential benefits of conducting

fundamental and applied research work, especially in enhanced heat

transfer, and finally as a true researcher to his soul, setting out to

obtain invaluable experimental data as well as insight into the

underlying mechanisms governing the related phenomena in

numerous fundamental problems. It is the best gift to us all from

Professor Bergles, that helped define many careers for many of us,

and produced and improved many products for the benefit of

mankind.

The experiments conducted by Professor Bergles not only

provide valuable data, but also bring out the importance of properly

designing an experimental set-up to obtain the desired data by

carefully controlling and limiting the influence of extraneous

variables. The approach taken by Professor Bergles shows the

thoroughness he applies in the design of the entire study covering a

specific topic. As seen from his studies on twisted tapes,

turbulators, microfins, and porous coatings, to name a few, he has

first considered a list of alternatives, and then narrowed it down to

specific configurations through broad experimental investigations.

This was followed by conducting well thought-out experiments to

reveal the most important parametric trends for the targeted

configuration, an art he has displayed time and again. From these

parametric studies, he has provided specific directions to the

researchers in academia as well as in industry. The generosity that

everyone has experienced while in his company is evident even in

his research work through extensive survey papers written by him

giving insights into the fundamental as well as applied aspects of

numerous heat transfer related problems.

A list of research publications by Professor Bergles published

through December 1996 is included under the list of references1.

The papers are classified into ten categories as shown in Table 1.

The numbers in front of a category indicate the paper reference

numbers.

1 The list of references is in a slightly different format.

Since there are many papers with the same authors in a given year of publication, the papers are numbered and are listed in a chronological order. The papers are referenced by these numbers in the table and the text (within square parenthesis).

TABLE 1. PUBLICATIONS BY PROFESSOR ARTHUR. E. BERGLES IN DIFFERENT RESEARCH AREAS

RESEARCH AREA

REFERENCES

Subcooled Boiling with Special Emphasis

on Cooling of High-Flux Components

4, 5, 18, 25, 33, 43

Fundamental Studies in Enhanced Heat

Transfer

6, 8, 10, 16, 17, 24, 30, 36, 40, 45, 47, 51, 53, 56, 59, 62, 64, 67, 76, 80, 85, 90, 91, 93,

99, 103, 104, 114, 115, 116, 117, 123, 127, 128, 130, 136, 138, 139, 145, 147, 148, 152,

155, 156, 157, 160, 161, 164, 165, 171, 172, 180, 181, 182, 191, 192, 193, 194, 196, 199,

205, 209, 212, 213, 214, 215, 222, 224, 225, 226, 228, 230, 231, 232, 233, 235, 236, 237,

238, 239, 246, 249, 250, 251, 252, 258, 261, 264, 265, 267, 268, 270, 273, 275, 277, 278,

279, 281, 282, 287, 288, 289, 290, 292 Fundamental Studies and Reviews of

Two-phase flow and Heat Transfer

9, 11, 12, 15, 20, 21, 22, 26, 27, 28, 32, 39, 50, 60, 70, 72, 73, 79, 96, 97, 100, 107, 112,

126, 131, 135, 143, 144, 146, 166, 188, 195, 241, 248, 253, 254, 257, 259, 260, 263, 266,

269, 280, 283, 293, 294, 295 Instability of Two-phase Flows

7, 13, 29, 31, 37, 38, 42, 44, 46, 48, 54, 71, 98

Heat Transfer to Laminar Internal Flows

23, 34, 41, 57, 61, 65, 69, 83, 106, 108, 118, 119, 120, 125, 129, 198, 286, 298

Flow and Heat Transfer of Refrigerants

(Including Evaporation and Condensation,

pure and oil-refrigerant mixtures)

158, 159, 170, 175, 183, 185, 187, 192, 204, 206, 207, 208, 210, 242, 243, 271, 284

(papers on enhanced tubes covered under Fundamental Studies in Enhanced Heat

Transfer) Reviews of Enhanced Heat Transfer

19, 35, 49, 52, 66, 74, 78, 84, 86, 88, 89, 92, 94, 95, 101, 102, 105, 109, 110, 111, 113,

121, 122, 132, 133,134, 137, 141, 142, 149, 151, 153, 154, 173, 189, 223, 229, 255, 276,

291, 296, 300 Studies and Reviews of Cooling of

Electronic Components

68, 81, 82, 140, 150, 162, 163, 166, 167, 168, 174, 176, 177, 178, 200, 201, 202, 203,

218, 219, 220, 234, 245, 256, 262, 272, 274, 299 General Heat Transfer

1, 2, 3, 14, 55, 58, 63, 75, 87, 169, 184, 190, 197, 216, 217, 221, 227, 240, 244, 247, 285,

297 History of Heat Transfer

77, 124, 179, 186, 211

2. Review of Research in Specific Areas

Table 1 covers all the three hundred papers published by

Professor Bergles. The following review presents the highlights

and some important details and relevance of his work. Due to

space constraints, all the paper listed in Table 1 could not be

discussed. Although figures and tables are not included here, the

readers can identify the relevant papers from the information

presented here, and then refer to those papers to find additional

details.

2.1 Enhanced Heat Transfer

Professor Bergles has been one of the most active proponents

of enhanced surfaces in heat transfer applications. He has

displayed a vision to recognize the importance of enhancement in

refrigeration, power, process, and microelectronic cooling

applications. In his 1979 paper on energy conservation via

enhancement (Bergles et al. [93]), he outlined the steps needed for

developing enhanced surfaces for commercial applications. As

stated in his paper, Commercialization represents the ultimate

stage of development; however, even commercial products require

additional development work. He is among the first ones to

comprehensively address issues related to the application of

enhanced surfaces - fouling, manufacturing and development cost,

and performance evaluation criteria for their selection. He

undertook the task of identifying the underlying heat transfer and

pressure drop mechanisms (for internal enhancement techniques)

for these enhancement devices, and provided insight which led to

further improvements. Vibration, ultrasonics, twisted tapes, bent-

strip inserts, finned tubes, microfin tubes, microporous surfaces,

microstructured surfaces, and stepped and wavy surfaces are

among the enhancement techniques he investigated, providing a rich

wealth of experimental data and a better understanding of the heat

transfer mechanisms associated with these devices. 2.1.1 Single-phase enhancement.

Vibration and Additives. Mechanical vibrations effectively

cause localized pressure fluctuations in the liquid adjacent to a heat

transfer surface. When the liquid is close to its saturation

temperature, enhancement is possible due to nucleation and

collapse of bubbles. Professor Bergles became interested in this

technique while working on the cooling of high-field electromagnets

at the National Magnet Laboratory at MIT (Bergles [6]). To

enhance the heat transfer to water flowing in the cooling channels,

the channel walls were subjected to vibrations. The localized

instantaneous reduction in pressure during a cycle resulted in

cavitation in the water at the channel walls. For wall temperatures

of about 30 C below the saturation temperature, the single-phase

heat transfer coefficient remained unaffected. However, as the wall

temperature approached the saturation temperature, the heat

transfer coefficient gradually increased, yielding up to 100 percent

increase. The enhancement was reduced as fully developed boiling

conditions were established at higher wall temperatures. The effect

of ultrasonic vibrations was tested further (Bergles and Newell [8])

with water flowing in annuli. They provided experimental results

in a parametric form to show the effect of system pressure,

annulus dimensions, vibrational intensity and wall superheat. The

presence of vapor in the flow channel drastically reduced the

enhancement, indicating the applicability of this technique only to

the subcooled region. The tubes were direct electrically heated in

the experiments, a technique that Professor Bergles used

extensively later with his in-tube research work to obtain local heat

transfer data.

Twisted tape and other inserts, and internally finned

tubes. Mechanical inserts and internal fins directly affect the

fluid flow field and the associated heat transfer process. Twisted

tapes and other in-tube inserts have been a major topic of

Professor Bergles research on enhanced heat transfer since 1969.

In his first paper on this topic, he reported a detailed experimental

study (Lopina and Bergles [16]) on heat transfer and pressure drop

with twisted tape inserts with water in fully developed turbulent

flow. The enhancement, as much as 100 percent, was attributed

primarily to the increased flow path, the increased circulation, and

the tape fin effect. An additive model was proposed to predict the

heat transfer coefficient from these mechanisms. The contribution

to heat transfer due to fin conduction was shown to be small, about

8 to 17 percent, for perfect contact between the tape and the wall.

For a constant pumping power, twisted tape inserts provided a 20

percent improvement in heat transfer over an empty plain tube.

Surface roughness and twisted tapes both provide

enhancement, although the mechanisms in the two cases are

different. The effect of combining these two techniques was

investigated (Bergles et al. [24]) in the turbulent region. Since the

two mechanisms do not overlap, the combination was expected to

provide further improvements, as was indeed the case. The

superposition technique worked well for correlating the heat

transfer data in spite of highly non-linear nature of the mechanisms.

This helped to clarify the discrepancies between two twisted tape

data sets obtained with the same geometry - attributable to the

differences in the surface roughness of the tubes.

The effect of brush and mesh type inserts was also studied

experimentally (Megerlin et al. [53]) for high heat flux applications.

Both inserts yielded dramatic improvements in heat transfer

coefficient, up to 1000 percent increase, as compared to plain

empty tubes. However, the pressure drop penalty was extremely

high, up to twenty times higher in certain cases.

Internally finned tubes are another form of enhancement

technique that were extensively tested by Professor Bergles

(Bergles et al. [36]). The heat transfer performance of eight

internally finned tubes was experimentally obtained under

turbulent flow conditions. The effect of roughness was found to be

insignificant for the internally finned tubes tested. These tubes

showed promise for a heat transfer performance improvement of

25 to 170 percent for a given pumping power.

The twisted tapes were tested for augmentation in the laminar

flow region (Hong and Bergles [64]) where a 1000 percent

enhancement in Nusselt number was confirmed with water and

ethylene glycol, covering Prandtl number ranges of 3-7 and 84-192,

respectively, with the two fluids. The experiments were conducted

for uniform-heat-flux boundary conditions in a 10.2 mm electrically

heated stainless steel tube employing twisted tapes with twist

ratios of 2.45 and 5.08. Hong and Bergles [64] developed a

correlation scheme for heat transfer and pressure drop that was

later extended to non-Newtonian fluids (Manglik et al. [222]). The

work of Hong and Bergles [64] was extended to include static-mixer

inserts and internally finned tubes (10 longitudinal fins, 1.57 mm

height, in a 14.2 mm tube) for process-industry applications

(Marner and Bergles [90]). These devices were found to provide a

higher heat transfer enhancement ratio compared to the pressure

drop penalty in the laminar region as compared to the turbulent

region where increases in pressure drop were significant. The

study was extended to laminar flow with Polybutene 20 (a liquid

polymer manufactured by Chevron Chemical Co., Prandtl number

range 1260-8130) by Marner and Bergles [147]. It was found that

the internally finned tubes yielded a 400 percent increase in heat

transfer coefficient for heating, while the twisted tape inserts were

more effective for the cooling applications, yielding 150 to 225

percent improvements over plain tubes. The internally finned

tubes yielded only marginal improvements during cooling.

The twisted tape insert results were analyzed (Manglik and

Bergles [182]) in an effort to develop a correlation to predict their

performance with laminar flow under uniform-wall-temperature

conditions. The experimental data on heat transfer indicated a

strong influence of five parameters: entrance effect, fluid viscosity

ratio (bulk to wall conditions), Prandtl number, tape twist ratio,

and swirl flow Reynolds number. The augmentation of highly

viscous laminar flow under constant-wall-temperature conditions

was investigated in subsequent papers (Marner and Bergles [231],

and Manglik and Bergles [261]), in which extensive experimental

data on heat transfer and pressure drop was reported.

The available experimental data for water, ethylene glycol, and

Polybutene 20 obtained in earlier studies were correlated within

25 percent (Manglik and Bergles [182]). However, this

correlation covered a limited range of parameters. In subsequent

papers, Manglik and Bergles [264, 265, 277] presented mechanistic

parameters to identify the effect of swirl on the flow field. The

balance of viscous, convective inertia and centrifugal forces is used

to predict the onset and intensity of swirl, as determined by the

swirl parameter. Based on this mechanistic description, four

regions are identified - viscous flow, thermally developed swirl

flow, swirl-turbulent transition, and fully developed turbulent swirl

flow. A continuous correlation covering these regions for uniform-

wall-temperature conditions was developed. The correlation

accurately represents the parametric trends, as well as the

asymptotic values for different variables.

Spirally-Grooved (Rope) Tubes. With the large amount of

heat transferred in power plant surface condensers, a tubeside

enhancement of the heat transfer coefficient could result in

considerable savings in the overall plant operation. Sp irally-

grooved tubes hold the promise of enhancing the heat transfer

coefficients on both sides; they are one of the most cost-effective

enhancement devices. Professor Bergles saw the need to develop a

good correlation scheme for these tubes for design purposes.

Rabas et al. [212] compiled a data bank of 458 data points from

five different sources. They proposed a new correlation scheme to

predict the heat transfer coefficient and friction factor for the

spirally-grooved tubes with an overall average error of less than 10

percent with the existing data. This represents one of the most

comprehensive correlation schemes which accounts for the

geometrical factors and fluid characteristics. One of the benefits of

this correlation is that it is possible to clearly see the parametric

influences of different geometrical parameters on the performance,

providing a valuable tool to the designer who is faced with the

selection of an optimum geometry based on not only the thermal,

but economic and manufacturing constraints also.

Turbulators for Fire-Tube Boilers Fire-tube boilers

employ high temperature gases flowing inside tubes. Since the heat

transfer coefficient on the outside is very high with boiling water, it

is desirable to increase the heat transfer coefficient on the gas side.

The overall objective in this application is to improve the boiler

efficiency. Other factors such as pressure drop, air-fuel ratio,

changes in the water side heat transfer coefficient, fouling, and

manufacturing cost are also important. In an experimental study

program, Junkhan et al. [138] and Bergles et al. [145] investigated

three commonly employed turbulators in fire-tube boilers (two

bent-strips and one twisted tape). The heat transfer enhancements

for these three inserts were measured to be 125 percent, 157

percent and 65 percent over a plain empty tube, while the

corresponding increases in pressure drop were 1100 percent, 1000

percent, and 160 percent at a Reynolds number of 10,700. The

width of the twisted tape was less than the tube diameter, and this

contributed to the lowering of its heat transfer enhancement to

about 50 percent of the next best tube, but the corresponding

pressure drop was reduced dramatically.

In order to identify the effect of the inserts on the flow

characteristics in a fire-tube boiler application, Nirmalan et al. [161]

conducted visual studies on seven different bent-strip types of

inserts. The heat transfer enhancement was measured to be

between 175 and 285 percent at a Reynolds number of 10,000,

with corresponding pressure drop increases of 400 to 1800

percent. The visual observations indicate that the flow disturbance

is most severe in the region where the bent strip comes in contact

with the tube wall. The flow remains relatively intact in the region

where the bent-strip does not touch the wall. Increasing the

number of contacting points would appear to increase the heat

transfer coefficient, however with a further penalty in pressure

drop. The perforated-strip insert indicates that the core region also

plays an important role in the heat transfer mechanism. The effect

of radiation between the strip in the core region and the wall was

also seen to play an important role, warranting further studies on

this aspect. In a subsequent study, Nirmalan et al. [164] tested

three additional inserts. They also addressed the issues raised in

their earlier investigation, Nirmalan et al. [161], by constructing

separate inserts representing the core region and the wall region of

the bent-strip insert. The results indicate that the inserts with

more rounded bends have a higher heat transfer coefficient as well

as a higher pressure drop penalty. The pitch was seen to play an

important role in the entrance region. The-core region insert was

shown to enhance the heat transfer much more than the wall region

insert, contrary to the earlier assumption that the core may not

play as important a role. However, the combined effect of the two

regions could be different from the individual influence of each

region. Nirmalan et al. [180] presented a theoretical model using a

surface renewal/penetration concept to develop a correlation

scheme for the bent-strip inserts. In this model, they assumed that

a packet of fluid is thrown toward the wall by the insert in the core

region. This fluid is heated by the wall during a transient

conduction process. The correlation scheme incorporates a

constant that is characteristic of the individual insert.

Ravigururajan and Bergles [251] also visually investigated the

flow phenomenon near the wall of ribbed tubes. Flow

visualization was seen as a useful tool in optimizing the ribbed

geometries.

Twisted tape Inserts with Non-Newtonian Fluids. Non-

Newtonian fluids are often encountered in chemical, petroleum,

food, biochemical, and pharmaceutical industries. Typical fluids in

these applications are paints, inks, soap and detergent slurries,

polymer solutions, greases, bitumen, paper pulp, corn syrup,

mayonnaise, and starch suspensions, which are pseudoplastics.

The three basic mechanisms of augmentation, (i) secondary flow

effects, (ii) an increased flow path, and (iii) fin effects, are still

responsible for enhancement in non-Newtonian fluids. Manglik et

al. [222] conducted an extensive study to investigate the heat

transfer and pressure drop for laminar flow of non-Newtonian

fluids in uniformly heated tubes with twisted tape inserts. The

experiments were conducted with two concentrations, 1.0 and 1.3

percent, of HEMC solution in a 12.85-mm-diameter stainless steel

tube. They attributed the increase in heat transfer coefficient with

pseudoplastics in single-phase flow to (i) the non-Newtonian

effects, and (ii) the variable consistency effects. Using the same

correction factors, the Hong and Bergles [64] correlation for the

uniform-heat-flux boundary condition was modified to predict the

heat transfer results within 30 percent. This is quite remarkable,

considering that the Hong and Bergles [64] could predict their own

water and ethylene glycol data to only within 25 percent. Similar

treatment resulted in a reasonable agreement with pressure drop

data as well [+25percent to -30percent].

Natural Convection. Natural convection heat transfer is an

important mode of heat transfer employed in many applications

including cooling of microelectronic devices. It is desirable to

extend its applicability to avoid the need for an active device such

as a fan or a blower in the cooling system. Augmentation of natural

convection heat transfer, therefore, has received renewed interest in

last decade.

A systematic study was undertaken by Professor Bergles to

investigate augmentation of natural convection heat transfer.

Bhavnani and Bergles [157, 239] conducted an interferometric

study of laminar convection heat transfer process from an

isothermal vertical plate with two types of transverse elements -

transverse ribs and transverse steps, placed horizontally across a

127-mm x 178-mm aluminum plate. A Mach-Zehnder

interferometer was used for taking local measurements. The effect

of pitch, height, and width (in case of ribs) was investigated. It

was found that the transverse ribs, in fact, decrease the overall heat

transfer rate by creating stagnation zones on both upstream and

downstream sides of the ribs. The stepped surfaces helped to

improve the performance. The effect of a sinusoidal wavy surface

was studied by Bhavnani and Bergles [213, 252]. This geometry

resulted in average heat transfer rates very close to plain-surface

values. There was an effect of wave amplitude seen in the results.

Smaller amplitudes caused the transition to turbulence at lower

Grashof number values of around 2x107 as compared to a plain

vertical surface. It was found that if the lower edge of the plate

was curved inside, it resulted in a better performance; however this

effect was not significant when two or more cycles of the wavy

surface were present along the plate length.

Fouling in Enhanced Surfaces. Fouling in heat exchanger

tubes is a major issue that needs to be addressed before enhanced

tubes can be employed, especially in critical applications such as

utility condensers. With this objective, Somerscales et al. [250],

and Bergles and Somerscales [290] carried out an extensive testing

program on tubes employing four types of enhancement

techniques, namely, the roped or corrugated, helical fins, axial fins,

and helical rib roughness. They conducted the tests with

magnesium oxide (approximately 3 m diameter) as the foulant

suspended in distilled water. The tests showed that the tubes with

axial fins, helical fins, and rib roughness exhibited a higher fouling

rate than a smooth tube under both high velocity and low velocity

tests, whereas the roped or corrugated tube showed a remarkably

lower fouling rate. However, Bergles [273, 278] reported a review

of other works in which the field testing of roped or corrugated

tubes showed considerably higher fouling rates with river and sea

water. They attributed the main reasons for this discrepancy to

the differences in the nature of the fouling elements present in the

laboratory testing and the field testing. The water in the field tests

contained dissolved salts, biological substances, finely divided sand

or silt, and other products of chemical reactions, while the

laboratory tests were conducted with a single foulant.

Performance Evaluation Criteria for Single-phase

Enhancement. Thermal equipment designers are often faced

with the task of selecting an appropriate enhancement device for a

given application. Many researchers were working on developing

guidelines to help in this selection process during 1960s and 70s.

Bergles [19] presented a comprehensive survey of different

augmentation techniques, and identified the need to establish

generally applicable selection criteria for augmentative techniques.

The factors such as development cost, initial cost, operating cost,

maintenance cost, reliability, and safety are important in this

selection process, but are too difficult to evaluate for general

application. The enhancement ratio in heat transfer coefficient, at

constant pumping power, length, and diameter, was used to

compare different single-phase enhancement techniques. In a

subsequent paper, Bergles et al. [45] proposed the eight

performance evaluation criteria for augmentation devices. The

parameters used in these criteria are - basic geometry, flow rate,

pressure drop, pumping power, and heat duty, while the three

possible objectives considered are - increase heat transfer, reduce

pumping power, and reduce heat exchanger size. With these

parameters, the following eight criteria were proposed - (i) for fixed

geometry and flow rate, increase heat transfer, (ii) for fixed

geometry and pressure drop, increase heat transfer, (iii) for fixed

geometry and pumping power, increase heat transfer, (iv) for fixed

geometry and heat duty, reduce pumping power, (v) for fixed heat

duty and pumping power, reduce exchanger size, (vi) for fixed heat

duty and pressure drop, reduce exchanger size, (vii) for fixed heat

duty and flow rate, reduce exchanger size, and (viii) for fixed heat

duty, flow rate, and pressure drop, reduce exchanger size. They

derived specific ratios for each criterion. To include economics, a

ninth criterion was introduced by comparing the total annual cost

with, and without, augmentation. These criteria have been

extremely helpful in convincing the heat exchanger industry of

potential benefits of switching to enhanced geometries.

Bergles et al. [59] further modified the performance evaluation

criteria to remove the assumption of constant temperature

difference between the hot and cold streams, and to include the

effect of the thermal resistances external to the enhanced surfaces.

Bergles et al. [62] applied these criteria in the selection of compact

heat exchanger surfaces. Webb and Bergles [123] presented

algebraic formulations of these criteria for low Reynolds number

flows. These criteria are now widely used in the development and

selection of compact heat exchanger surface geometries in

automotive, air separation, and many other industrial applications.

Applying these criteria to the the bent-strip inserts in fire-tube

boilers, Webb and Bergles showed that a favorable enhancement is

achieved in the Reynolds number range of 5000 to 30,000 under a

constant pumping power constraint, while the range drops to

between 3000 to 5000 under the constant pressure drop constraint.

2.1.2 Enhancement in Pool Boiling.

Vibration and Ultrasonic Techniques. The instantaneous

reduction in pressure in the liquid adjacent to a heated surface leads

to rapid growth and collapse of vapor bubbles resulting in

enhancement in subcooled pool boiling. Such effects of vibration

on the subcooled pool boiling heat transfer were studied by Bergles

[17] with water as the working substance. An increase in

vibrational energy markedly increases the pool boiling heat transfer

rates. Also noted was the effect of vibration on the CHF.

Park and Bergles [199] studied the effects of ultrasonics on the

heat transfer performance of a smooth pool boiling surface for

possible microelectronic cooling applications. They used

refrigerant R-113 as the test fluid. The results obtained were

similar to those obtained by Bergles [17] in that little enhancement

was observed for saturated conditions. Enhancement improved

with the subcooling. Burnout heat fluxes were not significantly

altered with the ultrasonics.

Low-Finned and Modified Structured Surfaces. The

meticulous work by Professor Bergles in revealing the nature and

mechanism of nucleate boiling in enhanced surfaces has been

outstanding. He combined experimental measurements with visual

techniques in revealing the flow of liquid in micropores and

channels of enhanced boiling surfaces. His work provided a clear

direction for industry in improving the performance of enhanced

pool boiling surfaces.

Low-finned tubes were used in pool boiling applications since

the 1940s. New surfaces with porous coatings and modified low-

finned tubes were manufactured commercially under the names

such as High Flux, ECR40, Thermoexcel-E, and GEWA-T. The

standard GEWA-K profile is a low finned surface, while GEWA-T

is a modified surface in which the GEWA-T fins are formed into a

T-shape by indenting a notch in the tip of the fin and then press-

rolling the tip. To understand the mechanisms responsible for the

higher performance of the GEWA-T surfaces, Ayub and Bergles

[156, 181] conducted an experimental study to compare the pool

boiling heat transfer rates for GEWA-T and GEWA-K surfaces.

Both surfaces showed more enhancement with R-113 than with

water, (maximum enhancement of 100 percent and 60 percent,

respectively, with R-113 and water). One of the highlights of the

performance of these tubes was the lack of a temperature

overshoot at the onset of nucleate boiling. Comparing the

performance of various geometries, Ayub and Bergles observed

that the gap width between the fins was an important parameter in

the thermal performance. The performance of a particular

geometry was dependent on the fluid employed - so the idea that

each geometry needs to be optimized for specific refrigerant was

proposed. The study of the flow pattern near the boiling surface

revealed that a predominant pattern of liquid inflow was present at

different locations. Liquid entered the finned surfaces at specific

locations while bubbles were generated at both sides of these

locations. A continuous liquid-vapor exchange mechanism,

different than the ones reported before for pool boiling, was

observed for these surfaces. They proposed a heat transfer model

which suggested that the heat transfer in this geometry is controlled

by latent heat transport and agitated natural convection. Professor

Bergles recommended that this study should be extended to CHF,

and the performance of this geometry studied in tube bundles.

This study represents a major step in the understanding and

subsequent development of enhanced surfaces in pool boiling

applications.

To enhance the performance of a GEWA-T surface further,

Ayub and Bergles [196] proposed to fill the gap between the fins

with a sponge material, polystyrene di-vinyl benzene polymer.

The presence of the sponge was expected to increase the bubble

activity in the gap. The pool boiling experiments were conducted

on these filled surfaces with distilled water. Experimental results

showed that the heat transfer rates increased by a factor of 1.5 to

2.3 relative to GEWA-T tubes with unfilled gaps.

Boiling hysteresis is caused by the fact that the superheat

needed to activate a cavity is higher than that required to keep it

active after it has been activated. Its effect is pronounced at the

onset of nucleate boiling where the higher superheat requirement

may cause the surface to overheat considerably before the pool

boiling is established. Also, a vigorous explosion on the heating

surface accompanies the onset of nucleate boiling in certain cases.

The effects of hysteresis were seen to be a major problem in

utilizing pool boiling in electronic cooling applications. Ayub and

Bergles [214, 237] conducted an experimental study to characterize

the hysteresis for GEWA-T surfaces. These surfaces exhibited a

small but different kind of hysteresis in which multiple smaller

excursions in wall temperature were observed during the transition

from natural convection to nucleate boiling. They attributed this to

improved natural convection in the low finned surfaces prior to

nucleation. The multiple excursions were believed to be due to the

isolation of nucleation sites from one another in the helical grooves.

Bergles and Chyu [104, 117, 127] present a study focusing on

the hysteresis effect of structured surfaces in pool boiling. They

showed that the tubes coated externally with porous metallic

coatings showed considerable nucleate boiling enhancement once

the nucleate boiling was initiated. However, similar to smooth

tubes, the enhanced tubes tested showed a hysteresis effect that

was not reported in any earlier literature. The hysteresis was

attributed to the wetting and flooding of the cavities by the liquid,

particularly for refrigerants. Bergles and Chyu [127] discuss the

problems encountered by temperature overshoot in different

industrial applications. The effect of pore size and the heat

transfer mechanism in tunnels formed by the microstructures are

discussed by Bergles [215] in a comprehensive paper on this

subject.

Professor Bergles extended the study of nucleate boiling with

water on enhanced surfaces to pure refrigerants R-113 and R-11

and their binary mixtures as reported in Trewin et al. [282]. The

tubes tested included smooth, knurled (Turbo-B), and sintered

(High Flux) surfaces. Nucleation on these surfaces was initiated in

some cases with a wall superheat of less than 0.1 C. The

hysteresis effect was most pronounced for small porosity High

Flux surfaces, resulting in an overshoot of 10 C. The porosity of

the High Flux surface played a major role in the heat transfer

process. Smaller porosity tubes performed the best (after the

nucleate boiling was initiated following the hysteresis effect) among

all the tubes tested. Another major conclusion of the study was

that the sizes of the channel openings in Turbo-B tubes had very

little influence on the heat transfer rate, indicating that the

subsurface channel itself plays an important role. They identified

thin film evaporation inside the channels as the main heat transfer

mechanism in the Turbo-B tubes. The pool boiling heat transfer

coefficients with mixtures showed a degradation for all three

surfaces, although the enhanced surfaces consistently performed

better than the smooth tube. A need for developing better

correlating schemes for mixtures with enhanced surfaces was

identified.

2.1.3 Enhancement in Two-Phase Flow, Boiling and

Condensation. There is a need for improving the

performance of heat transfer equipment incorporating boiling and

condensation processes from an energy conservation viewpoint.

The benefits to the refrigeration, power and process industry result

directly in overall conservation of energy resources. With the rapid

advancements in the enhancement techniques for single-phase heat

transfer, it was only a matter of time before it was realized that

further improvements in flow boiling and condensation heat

transfer rates are warranted to improve the overall efficiency of the

thermal systems. Professor Bergles addressed this need by

conducting extensive research on enhancement in flow boiling and

condensation.

Doubly Rippled Surface for External Condensation.

One of the most important factors in determining the external

condensation heat transfer is the existing film thickness on the

condensing surface. This layer presents a thermal barrier to heat

transfer. In applying the pool boiling mode to electronic cooling,

efficient condenser surfaces were needed to transfer the heat from

the condensing refrigerant to the cooling water. In 1972,

Markowitz et al. [47] developed a doubly rippled surface; the

main ripples help to drain the condensate film effectively from the

downward facing condenser surface, while the secondary ripples

reduce the film thickness on the condenser surface between the

main ripples. An analytical formulation was presented to predict

its thermal performance by extending the laminar film condensation

theory. Although the experiments yielded a lower heat transfer

rates than predicted by the theory, a number of practical problems

arising in such research work were pointed out. These include the

proper degassing procedure, the effect of non-condensables, and

the assumption of non-uniform heat flux over the condensing

surface.

Twisted tapes, Internally Finned Tubes, Wall Roughness Elements and Microfins for In-Tube

Evaporation and Condensation. The work of Professor

Bergles on twisted tapes in single-phase flow has provided an

insight into the heat transfer mechanism, and a powerful correlation

technique along with invaluable experimental data for this

geometry. Professor Bergles saw the potential of twisted tapes in

in-tube condensation application, and undertook a detailed study to

explore this field. Although condensation enhancement was

studied by many investigators, Professor Bergles pointed out that

very few efforts were directed toward the in-tube enhancement.

Royal and Bergles [67, 85] conducted a detailed experimental

study on the augmentation of in-tube condensation of low pressure

steam in horizontal tubes by means of twisted tapes and internally

finned tubes. Twisted tapes showed an improvement of 50

percent while the internally finned tubes showed an improvement

of 300 percent in heat transfer rates over empty smooth tubes. To

make the data useful to practitioners, Royal and Bergles presented

correlations for both geometries, using their own data as well as

other data available in literature.

The work on in-tube condensation with water was extended to

refrigerants by Luu and Bergles [99] for application in refrigeration

and air-conditioning. Twisted tape inserts and three different

internally finned tubes were tested. Internally finned tubes

performed better than twisted tapes. Using performance criteria

with a constant pressure drop, internally finned tubes were by far

superior, and held promise in commercial applications. Professor

Bergles, however, pointed out that the fin geometries resulting in

optimum performance for refrigerants are different than those for

water. Luu and Bergles [103] presented qualitative reasons for the

differences in condensation characteristics of water and R-113.

The important parameter is the condensate film thickness, which

depends on the surface tension, density ratio of the two phases and

the wall shear stress. The twisted tapes were seen as possible

retrofit devices in existing condensers.

The wall roughness elements, such as helical repeated rib and

spirally fluted tubes, were found to be effective in enhancing the

single-phase heat transfer. Professor Bergles investigated their

performance for in-tube condensation. Luu and Bergles [ 114] and

Bergles [139] report that their experimental study on helical

repeated ribs and the spirally fluted elements yielded 80 percent

and 50 percent enhancement in the heat transfer coefficient over a

smooth tube. Correlations for these geometries were proposed.

The use of microfin tubes for condensation application was

extensively studied by Professor Bergles. Khanpara et al. [171]

compared the performance of one plain and eight microfin tubes for

in-tube condensation of R-113. The heat transfer coefficients

improved considerably over smooth tube values. The main reasons

for the enhancement during condensation were identified as the area

increase due to fin effect, thinning of the condensate film, and the

disturbances caused by the presence of fins. The effect of fin

shapes, fin height, number of fins, and spiral angles were discussed.

This information is useful to the manufactureres in the design of

new and more efficient microfin geometries.

Microfin tubes were being introduced in refrigeration industry,

and with the relatively little flow modification in the bulk flow,

they offered a high heat transfer coefficient coupled with a low

pressure drop penalty for evaporatiors as well. Khanpara et al.

[165] conducted an extensive study on one plain and eight microfin

tubes of different geometry to arrive at the optimum performing

tube. They conducted experiments in electrically heated test

sections over a range of quality, mass flux and heat flux. The result

clearly identified the tube which performed best for refrigerant R-

113 over the given range, and changes in the microfin geometry

were proposed based on the observed trends.

Khanpara et al. [183] also conducted a study comparing the

electrically heated and fluid heated test sections during evaporation

of refrigerant R-113 in smooth and microfin tubes. The heat

transfer coefficient was same for the two cases at low and medium

mass flow rates; however for high mass flux rates, the electrically

heated long test section gave 20 to 40 percent higher heat transfer

coefficients. Further investigation is needed in this area.

Comparing various enhancement techniques proposed in the

literature, Professor Bergles conducted a systematic study to

evaluate their performance with refrigerants. Reid et al. [191, 249]

compared the performance of five microfin tubes, and a smooth

tube with a twisted tape insert, with the performance of two

different diameter smooth tubes. The heat transfer coefficients and

pressure drops were obtained for these geometries over a wide

range of quality, mass flux, and heat flux. This work showed that

the microfin tubes with helix angle around 16 to 18 degrees

performed well, with a low increase in pressure drop.

The effect of fluid properties on the performance of microfin

tubes of different geometries is an important area in refrigeration

application. Khanpara et al. [192] compared the performance of

different microfin tubes with R-113 and R-22 refrigerants over the

range of operating conditions commonly encountered in

refrigeration practice. The enhancement in heat transfer was similar

with the two refrigerants in the high mass flow region. In the low

mass flow region, enhancement was higher with R-113. They

emphasized the need to develop a correlation scheme for microfin

tubes. Schlager et al. [230] present a detailed study on evaporation

and condensation heat transfer in microfin tubes with R-22. The

microfins showed considerable enhancements (factors of 2.3 to 1.6

for evaporation, and 2.0 to 1.5 for condensation). Corresponding

pressure drop increases were only 20 to 40 percent. The effect of

tube diameter on the performance was insignificant. This shows

that a microfin geometry could be applied to different diameter

tubes without any modifications. These studies clearly indicate the

superior performance of microfin tubes in boiling and condensation.

It is therefore no surprise to see their widespread use in

refrigeration and air-conditioning industry.

The performance of several microfin tubes in a fluid heated test

set-up was tested with Refrigerant R-22 by Schlager et al. [225].

The tests indicated that the performance of all microfin tubes

appeared to be closer together. An increase in mass flow rate

decreased the evaporative performance. The pressure drop penalty

was less than the heat transfer increase, but it increased with

increasing mass flow rate.

In practical applications, small amount of oil is generally

present in evaporators and condensers of a refrigeration system.

Schlager et al. [204] present a detailed study on the effect of oil on

the evaporation and condensation heat transfer in a low-fin tube.

Refrigerant R-22 was used with a 150-SUS naphthenic mineral oil.

Small amounts of oil, below 1.5 percent, led to an improvement in

the evaporative heat transfer coefficient for smooth tubes, but the

low-fin tube showed very little enhancement. Higher quantities of

oil degraded the evaporator performance for the low-fin tube below

the smooth tube level. The condensation performance degraded

with the presence of oil, but it was less adversely affected

compared to smooth tubes. The work clearly showed that the

presence of oil in refrigeration systems affects the thermal

performance of augmented tube evaporators and condensers.

A similar study was conducted by Schlager et al. [209] to

investigate the effect of oil on the evaporation and condensation

heat transfer for smooth and microfin tubes. As found in earlier

studies, the presence of oil improved the evaporation heat transfer

coefficient of smooth tubes. Microfin tubes also exhibited similar

trends, although the enhancement was less. The condensation heat

transfer coefficient decreased with an increase in oil concentration

for both tubes. They also discussed specific effects of oil

concentration and mass flux. Subsequently, Schlager et al. [224,

233] found that the effects of 300-SUS oil were similar to those

with 150-SUS oil.

Schlager et al. [228, 238] presented the results showing the

effect of oil on the heat transfer and pressure drop performance of

smooth and internally finned tubes with R-22. The performance

trends of the finned tubes were similar to those for the microfin

tubes, but were consistently below the microfin tubes. During

condensation, both enhancement techniques resulted in lower heat

transfer rates, as compared with the smooth tubes with the

addition of oil.

Schlager et al. [235, 236] conducted a detailed literature survey

and presented design-correlations for predicting the heat transfer

coefficients with refrigerant-oil mixtures during evaporation and

condensation inside smooth and microfin tubes. These correlations

are extremely useful to the designers of the heat transfer

equipment.

The mechanisms responsible for degradation of heat transfer

performance in microfin tubes with oil were not clearly understood.

Ha and Bergles [270] conducted a careful study to investigate the

effect of oil using visual observations and careful mass fraction

measurements in the liquid film near the wall. They found that an

oil rich layer adhered to the wall, and its thickness increased with

oil concentration and mass flow rate. They identified the thermal

resistance of this layer as the primary reason for the performance

degradation.

Twisted tapes in Dispersed-Flow Film Boiling The swirl

flow generated by twisted tapes could be effective in modifying the

film-flow and heat transfer behavior in the dispersed-flow film

boiling region. Bergles et al. [30, 40] conducted an experimental

study to validate these findings experimentally. Their results show

that up to 200 percent enhancement is possible with the

introduction of a swirl generator in the flow. In this work as well,

Professor Bergles considered the practicality of the enhancement

device by comparing its performance under a given pressure drop

or pumping power condition. Assuming that the swirl flow

promotes thermal equilibrium in the two-phase flow, a model was

proposed that requires only one adjustable constant, the fraction

of the tube wall covered by the centrifugal droplets. With an

optimized constant, the correlation described the data well.

Enhancement in Film Evaporation

Horizontal spray-film evaporators are employed in

desalination, refrigeration, and chemical process operations. Their

applicability to ocean thermal energy conversion systems was

evaluated by Chyu et al. [130]. Since the ocean thermal energy

systems work between small temperature differences, improving

the performance of the evaporation and condensation processes in

the power cycle is critical. In the evaporator, the nucleate boiling

in the film would be important, and porous and microstructures,

employed in pool boiling enhancement, are strong candidates.

Chyu et al. tested five surfaces and found a considerable

improvement over smooth surface performance. However, the

performance with spray was below the corresponding pool boiling

performance for these surfaces. They attributed the main reason

for the poor performance to the unfavorable temperature profiles in

the film.

The enhancement with the structured surfaces in falling-film

evaporators was investigated by Chyu and Bergles [148, 232]. The

surfaces tested include smooth, Wieland-Werke Gewa-T deformed

low fin surface, Hitachi Thermoexcel-E tunnel-pore surface, and

Union Carbide Linde High Flux porous metallic matrix surface.

Falling-film evaporation over smooth surfaces yields higher heat

transfer coefficients than the corresponding pool boiling values.

The falling-film results for structured surfaces approach the pool

boiling results over structured surfaces at high heat fluxes. Distinct

effects were seen in the convective and nucleate boiling mechanisms

depending on the surface tested. Effects of film flow-rate and

liquid feed-height were of secondary importance. The need was

emphasized for investigating the structured surfaces with different

fluids for specific applications.

2.2 Review Papers on Enhanced Heat Transfer

One of the most significant contributions made to the technical

community by Professor Bergles is in providing with state-of-art

reports in many areas, including enhanced heat transfer. He started

his work in this area in early 60s, and is still in the midst of

publishing various review papers.

His first elaborate review paper on augmentation techniques

appeared in 1969, Bergles [19]. He referenced 371 papers in this

work, and classified them into following categories: vortex flows,

including twisted tape swirl generators; vibration of the heater

surface; electrostatic fields; and various types of additives. The

non-boiling, boiling, and condensation in free and forced

convection, and mass transfer in forced convection were covered.

The review included key information from different papers, and

offered guidance for practical applications by presenting turbulence

promoter data in terms of a pumping power performance criterion.

He reported important experimental data in figures, which were

carefully drawn to include detailed information on the experimental

conditions for which the results are presented. He compiled and

presented the experimental investigations in a tabular form to bring

out clearly their key features. Through this paper, Professor

Bergles raised the standard for presenting the state-of-art review

papers, and he himself wrote more than fifty such in-depth review

papers on different aspects of heat transfer.

To aid the researchers in narrowing down their search to

specific papers, and to help designers find specific references in

their field of interest, Professor Bergles started preparing a

bibliography of available literature on different topics. Bergles and

Webb [35] presented the first such bibliography on augmentation

of convective heat transfer. It included references to 472 papers.

Professor Bergles then developed an extensive bibliographic

collection, resulting in a six-part paper series coauthored with

Professor Ralph Webb - [86] and [92] in 1978, [94] and [95] in

1979, and [102] and [105] in 1980. Even with the availability of

the computerized on-line services, the exhaustive bibliographic

collections, presented under specific categories, are valuable

resource for researchers and designers since a computerized search

is able to catch only a fraction of the available literature.

Professor Bergles kept pace with the developments in the

enhanced heat transfer, and provided critical surveys, which were

valuable in determining the potential of a given augmentation

technique for a specific application. He constantly updated his

reviews on augmentation, and published them periodically since

1969. Reviewing the augmentation of convective heat transfer, he

has authored or coauthored the following papers - Bergles et al.

[49], [52], [66], [84], [88], [89], [109], [110], [111], [132], Bergles

et al. [149], [151], [173], and [189]. References [153] and [154],

published in 1986, deal with enhancement in high temperature

applications. A major part of Professor Bergles research activity

was directed toward the enhancement in boiling and condensation

applications. He presented his first paper in this area, Bergles [74]

in 1976, and has steadily reported latest compilation of research

work - [78], [134], [142], and [229].

Professor Bergles presented extensive review papers on the

effects of temperature-dependent fluid properties on laminar flow

heat transfer [119, 120] and enhancement techniques in the laminar

flow region (Joshi and Bergles [129]). In laminar flow

enhancement, his review papers, Joshi and Bergles [113] and

Bergles and Joshi [122], provide an extremely valuable resource for

selecting a specific type of enhancement device, and understanding

the underlying enhancement mechanism occurring in it.

Professor Bergles classifies the enhancement techniques,

implemented in last twenty years or so, as second generation heat

transfer technology. Starting with the smooth tube as the first

generation, the finned surfaces and the 2-D structured surfaces are

classified under second generation enhancement technology.

Starting in 1983, Professor Bergles has extensively reviewed the

second generation enhancement devices in the following papers -

Webb and Bergles [137], Bergles and Webb [141], [223], [255],

[276], [291], and [296]. The current thrust of Professor Bergles

work, as described in his recent paper, Bergles [300], is toward the

third generation enhancement technology that includes 3-D

roughness elements, 3-D fins, microfins, and metallic matrices.

Although some of these techniques have been invented many years

ago, their wide-spread acceptance in industrial application really

determines their age.

2.3 Laminar Internal Flow

Professor Bergles started his work on laminar internal flow

with an extensive study of the effect of natural convection on heat

transfer, in fully developed laminar flow of water inside a tube,

with uniform heat flux at the wall (Newell and Bergles [23]). This

study included the effects of the circumferential variation in the

wall temperature by considering two limiting tube-wall conditions -

infinite-conductivity tube, and glass-tube (having the same thermal

conductivity of the wall material as the test fluid, water). At low

Reynolds numbers, a secondary flow due to natural convection is

established, which is symmetrical about the vertical plane passing

through the axis of the tube. The flow field is three-dimensional,

spiraling, in character. The governing differential equations

employed stream functions, and were solved using a finite

difference formulation. Results were presented in terms of

detailed parametric relationships. To make the results useful to

designers, correlations for Nusselt number, and a pressure drop

parameter, (friction factor Reynolds number) were presented as

functions of bulk temperature, heat flux, and tube radius.

Computer limitations did not permit extensive solutions with

secondary flows in the entrance region. Bergles [34], in a later

technical note, discussed the applicability of different assumptions,

such as constant wall temperature, Prandtl and Reynolds number

effects, and the entrance region effect.

After analyzing the combined convection problem analytically,

Professor Bergles undertook the experimental work to verify the

numerical results. Bergles and Simonds [41] conducted

experiments with electrically heated, coated glass tubes, using

water as the test fluid. The final correlation, presented in a

graphical form, covered both, the developing and the fully

developed flow regions. The heat transfer results were much higher

(about 3 times higher for a Rayleigh number of 106 in the fully

developed region) than the corresponding constant property

solution. In this work, Professor Bergles has shown a mastery in

designing experiments to obtain meaningful information regarding a

phenomenon, while providing useful design correlations to

engineering practitioners. We see this throughout his experimental

work in many different areas.

Hong et al. [57] extended the numerical and experimental work

to combined convection in electrically heated metal tubes. Their

results agree with theoretical analysis; the results for the metal tube

lie between the constant heat flux and the constant wall

temperature cases. A correlation was presented for Nusselt

number by including a parameter representing the ratio of the fluid

to wall thermal conductivities. Morcos and Bergles [61] included

the effect of variable properties in the laminar fully developed

region. The mean film temperature was employed to account for

the property variations rather than a viscosity correction factor.

Hong and Bergles [69] presented analytical solutions for the

combined convection with fully developed laminar flow in a

circular tube by considering the temperature-dependent viscosity.

The results were then correlated in simple forms to cover a wide

range of parameters. The results with variable properties lie 50

percent above the results for the constant property solution.

To gain a further insight into the heat transfer mechanism with

twisted tape inserts, Hong and Bergles [65] studied the laminar

heat transfer in the entrance region of a semicircular tube with

uniform heat flux. They later employed the results of this work in

the models developed for twisted tape inserts. Hong and Bergles

[83] present the analytical solutions for developing and developed

flows, and show that the heat transfer rate is increased by 200

percent, and the entrance region is reduced to one-tenth, by

including the variable property effects.

Joshi and Bergles [106, 108, 125] analyzed laminar flow heat

transfer in circular tubes, with uniform wall heat flux, for non-

Newtonian fluids. They compared the results of the analytical

study with available correlations. Using their own experimental

data covering a broad range of parameters, they presented two

correlations based on the temperature dependence of the rheological

characteristics of the fluid. Joshi and Bergles [118, 129] extended

the study to the uniform wall temperature case.

The papers by Professor Bergles on enhancement in laminar

region are summarized in Section 2.5 under review papers.

2.4 Heat Transfer to Refrigerants (Boiling and Condensation Heat Transfer)

A major part of Professor Bergles research work has been

directed toward the refrigeration industry. His work on the

enhanced tubes (especially microfin tubes) for boiling and

condensation is noteworthy, and is covered under section 2.1.3. In

this section, his work on other aspects of heat transfer to

refrigerants is covered.

Although much of the research in academia is directed toward

pure refrigerants, most refrigeration systems employ oil refrigerant

mixtures to provide lubrication to the compressor in the system.

With fluorinated hydrocarbon refrigerants, oil is soluble in

refrigerant, and is carried over from compressor to condenser and

evaporator. Baustian et al. [158] report a study summarizing

predictive methods for thermophysical and transport properties of

oil-refrigerant mixtures. To determine the oil concentration in the

mixture, Baustian et al. [159, 170] reviewed different electrical and

optical properties as possible bases for real-time measurements.

They recommended two types of measurements - capacitance

measurement and refractive index measurement. Continuing this

study into the experimental phase, Baustian et al. [206, 207, 208]

built and tested three concentration measuring devices based on the

density, viscosity, and acoustic velocity respectively. These

devices provide practical solutions in the refrigeration industry for

on-line measurement of oil concentrations.

Continuing with the practical problem of oil-refrigerant

mixtures, Manwell and Bergles [242] presented an experimental

study of gas-liquid flow patterns with Refrigerant R-12. They

conducted the study with smooth and micro fin tubes. The

presence of oil caused foaming, which wetted the walls, and formed

foamy slugs in the evaporator. This explains the improvement in

the heat transfer coefficient with addition of oil to pure refrigerants

in smooth tubes. Since the wetting phenomenon is already present

in micro fin tubes, the presence of oil does not necessarily improve

the heat transfer. Further, they did not observe the foaming

behavior in microfin tubes. This study seems to be the first one to

address the mechanism of enhancement with oil-refrigerant

mixtures in smooth and microfin tubes.

The oil concentration in evaporator and condenser plays an

important role in the heat transfer mechanism. Schlager et al. [243]

measured these oil concentrations as functions of heat and mass

fluxes, and exit superheat. As expected, with the exiting refrigerant

closer to saturation, the oil concentration in the evaporator

increased. The experiments showed that the concentrations in the

evaporator were as much as three times, and those in the condenser

were about twice the average concentration in the system.

Professor Bergles conducted extensive heat transfer

measurements in evaporators and condensers with oil in smooth

and microfin tubes. This work is reviewed under section 2.1.3

under enhancement in two-phase flow.

Stratification effects in horizontal evaporators cause

circumferential variation in heat transfer coefficient. Ha and

Bergles [271] conducted a detailed experimental study to measure

this variation as a function of other system parameters. The effect

of axial wall conduction influenced the heat transfer coefficient by

only 10 percent. In runs with clearly separated flow, the heat

transfer coefficient at the base was 3-5 times higher than the

average value. The importance of liquid film for evaporation is

confirmed, indicating severe deterioration in heat transfer in the

upper part of the tube exposed to vapor in the stratified flow.

Ha and Bergles [284] present a valuable discussion on the effect

of the type of heating on the heat transfer mechanism in boiling

systems. They compared electric resistance wire heating, direct

electric heating, and liquid heating, and listed advantages and

disadvantages of each method. The paper provides valuable insight

on the heat transfer mechanism in smooth and microfin evaporator

tubes, with pure refrigerant and oil-refrigerant mixtures. The

dryout toward the exit of the evaporator is delayed with microfin

tubes, resulting in a significant increase in the heat transfer

performance of these tubes.

2.5 Fundamental Studies and Reviews of Two-phase Flow and Boiling Heat Transfer (Including Boiling, and Two-phase flow instabilities)

Professor Bergles addressed many current issues in two-phase

flow, boiling heat transfer, and CHF under different configurations

- pool boiling, subcooled flow boiling, and saturated flow boiling.

To cover his contributions, his publications in these two broad

areas are presented under the following specific subsections.

2.5.1 Two-phase Flow Regimes and Flow Structure.

Flow patterns in two-phase flow were studied by early

investigators with air-water, and oil-gas systems under adiabatic

conditions. To understand the heat transfer in high pressure boilers

applied to the nuclear industry, Bergles and Suo (9) undertook an

experimental study to identify the flow patterns under diabatic

conditions. They investigated the effect of tube length, system

pressure, mass flux, and inlet subcooling in vertical upflow. They

identified the flow regimes primarily with an electrical resistance

probe. They also took high-speed still pictures, but the resistance

probe was found to be more useful in establishing different flow

patterns. Changes in pressure, tube length, and inlet temperature

significantly affected the flow regime boundaries. Bergles et al.

[11] conducted a similar study with low pressure water, and

developed composite flow pattern maps to illustrate the effects of

pressure, length, and inlet temperature on the flow regime

boundaries. Focusing on the spray annular regime, Bergles and

Roos [15] measured the film thickness, and obtained the first

evidence of smooth dryout at low velocities. The film produced a

fluctuating signal in the electrical probe, pointing to a possibility of

nucleation, or entrained vapor, in the film close to the dryout

conditions.

Professor Bergles realized the importance of two-phase flow in

rod bundles as applied in nuclear steam generator application.

Bergles [26] investigated the two-phase flow structure

visualization with high pressure water in a rod bundle, and found

significant differences in flow patterns in different subchannels.

Using the electrical resistance probe, he measured the film

thickness in the subchannels, and reported extensive data on flow

regimes as a function of quality and mass velocity. Significant

differences were also reported between the diabatic and adiabatic

conditions. The flow regime boundaries were shifted to lower

quality with heat addition. The electrical probe was thus seen as a

useful tool in sensing an imminent CHF condition.

Another aspect investigated by Professor Bergles was the two-

phase critical flow under diabatic conditions, which is relevant in

studying the accident conditions in nuclear reactor safety analysis.

Bergles and Kelly [27] conducted experiments with water, and

found that for qualities below 0.04, the earlier models developed

for diabatic flow underpredicted the flow rate.

2.5.2 Two-Phase Flow Mechanism, and Instabilities.

Evans et al. [20, 32] studied the propagation of shock waves in

different two-phase flow regimes with air-water flows. The

presence of entrained liquid mist was confirmed to have an

enormous effect on the pressure wave propagation, and little or no

acoustic energy was transmitted through the liquid film. The flow

regimes, such as slug flow and annular flow, influenced the pressure

wave propagation considerably. This fact explained some of the

discrepancies in the data reported earlier in literature. Yadigaroglu

and Bergles [31] conducted experiments with Freon-113 to study

the density wave oscillations, and observed higher mode

oscillations, transmitting at a fraction of the transit time through

the channel. They also presented a stability map to exp lain the

phenomenon.

2.5.3 Instrumentation in Two-phase Flow Professor

Bergles refined the art of experimentation by using many new

instrumentation techniques. In one of his papers, Bergles [21]

presented an excellent survey of electrical probes in the study of

two-phase flows. He described the core-wall conductivity probe

used in determining the flow pattern, void fraction, and liquid film

thickness. This study provides a very useful source to anyone

who wants to develop these probes. Also, he compared the

accuracy of measurements of the electrical probes with other

techniques.

More recently, Bonetto et al. [253] used a hot wire

anemometer, and developed a probability density function to

obtain the information regarding void fraction, bubble size, and

vapor velocity from flow boiling experiments. Carvalho and

Bergles [254] further applied the hot wire anemometer to measure

the local void fractions in pool boiling of FC-77 over small vertical

heaters, simulating immersion cooling of electronic chips. The low

contact angle of FC-77 yields in a more satisfactory discrimination

between the two phases. They also found the optimal sensor

temperature corresponding to 60C, which was much higher than

those reported in earlier studies.

2.5.4 Pool Boiling Heat Transfer. Pool boiling heat

transfer data is generally obtained under steady-state conditions.

Thompson and Bergles [28] investigated the applicability of the

pool boiling curve to quenching problems. They found large

differences between the quenching data and the predictions from

pool boiling correlations. The presence of surface deposits on the

material being cooled disturbed the vapor film and caused early

transition to nucleate boiling, thereby reducing quench times below

the conventional boiling predictions. Further, it also implied that

the transient techniques are not suitable for obtaining the steady-

state pool boiling curve.

Another major factor affecting pool boiling data in industrial

applications is the presence of contaminants. Jensen et al. [97]

experimentally studied the effect of Cosmoline, JP-4, turbine oil,

and phosphate on the pool boiling curve. The presence of

Cosmoline improved heat transfer rates, the highest coefficient

being obtained at the highest concentration tested (1000 ppm).

However, DNB occurred at lower heat fluxes compared with

distilled water. JP-4, on the other hand, had no influence on heat

transfer or DNB. Turbine oil produced erratic results, sometimes

causing explosive bubble formation on the heater surface. At high

concentrations, the heat transfer results were dramatically below

the distilled water curve. DNB was also decreased with the

addition of turbine oil. Addition of phosphates generally shifted

the contaminant pool boiling curve back to normal, though the

DNB occurred at the same level as with the contaminants. The

orientation, vertical or horizontal, did not affect the boiling

characteristics with or without contaminants.

Carvalho and Bergles [283] studied pool boiling over small

vertical heaters, similar to electronic chips, and identified different

regimes, rogue sites, incipient boiling, patchy nucleate boiling, fully

developed nucleate boiling, and vapor coalescence (leading to dry

patches). Using a hot wire anemometer, they obtained void

fraction profiles near the heater surface as a function of heat flux.

They established the formation, and subsequent propagation of dry

patches as the mechanism leading to CHF in pool boiling.

2.5.5 Subcooled Flow Boiling Heat Transfer. In one of

his first papers, Professor Bergles presented an often referenced

paper on the forced convection boiling heat transfer with Professor

Rohsenow, Bergles and Rohsenow [4]. They analyzed flow boiling

heat transfer with subcooled and saturated liquids, and presented a

criterion to determine the size ranges of nucleating cavities for a

given superheat and flow conditions. Also, the heat transfer rates

in the region between the forced convection and the fully developed

boiling is interpolated using the inception point as the starting

point on the line representing forced convection heat transfer, and

merging with the fully developed boiling curve. This inception

condition is still widely used in the current literature in many

different geometries, from smooth tubes to complex ink jet printer

heaters.

Bergles and Dormer [18] conducted extensive experiments to

study the pressure drop in subcooled boiling of low pressure water

in 2.5-4.0 mm diameter tubes. The pressure drop data was then

correlated in a chart form, and curves were presented to cover the

entire data. This was one of the first studies in this area. The

information is useful in studying stability of multichannel systems

as well.

Professor Bergles studied the nucleation phenomena in

subcooled boiling systems, and noted that a larger amount of

superheat is needed for a given cavity than predicted from

theoretical considerations. Murphy and Bergles [43] attributed

this effect to the dissolved gases that increased the total pressure in

a cavity. However, it was found that with fluorocarbon systems,

large superheats were required to initiate nucleation. This caused

the hysteresis effect, which they attributed to the total flooding

of the cavities with low contact angle fluids, such as fluorinated

refrigerants. The commercially available porous surfaces tend to

prevent the deactivation of the cavities.

Vandervort et al. [266] studied the subcooled flow boiling of

water in a 2 mm diameter tube under high heat flux boiling. They

observed streams of small diameter bubbles (estimated to be 3 m)

at the exit section of the tube. They presented a detailed

description of the forces acting on the bubble and the associated

heat transfer mechanism. They believed that Marangoni force was

the dominant force, followed by surface tension and drag. The

discussion presented in the paper provides a good basis for

developing a mathematical model describing subcooled boiling heat

transfer near CHF.

Tong et al. [294] investigated pressure drops in small diameter

tubes with subcooled flow boiling of water. The earlier work by

Bergles and Dormer [18] was extended with 1.05-2.44 mm diameter

stainless steel tubes. The subcooled boiling pressure drop was

found to be directly proportional to mass flux and length to tube

diameter ratio, but inversely proportional to the tube diameter.

They developed a pressure drop correlation which is particularly

useful in designing cooling systems to accommodate high heat

fluxes.

2.5.6 Flow Boiling Heat Transfer in Enhanced Tubes.

This is covered earlier in the section on enhanced heat transfer,

Section 2.1.3.

2.5.7 CHF in Pool and Flow Boiling. CHF studies are

important in designing flow boiling systems for cooling high flux

systems, such as electromagnets. These devices use narrow

diameter passages due to space restrictions. Much of the CHF

data in literature pertained to large diameter tubes. To close this

gap, Bergles [5] undertook a detailed experimental plan to generate

data on CHF for flow of water in 1.5 to 4 mm diameter, electrically

heated, stainless steel tubes. Small diameter tubes were found to

give a higher CHF than large diameter tubes, making them

especially suitable for high-flux cooling systems. Flow oscillations

due to an upstream compressibile volume was found to reduce the

burnout heat flux considerably. Earlier studies which recorded a

lower CHF were believed to be affected by this problem. Bergles

et al. [11] and Bergles and Kelly [27] conducted additional

experiments with subcooled water at low pressure. Choked flow

was found to be prevalent under these conditions. CHF was found

to be a complex function of both local and inlet conditions.

High pressure water is used in power generation systems, and

CHF data is needed in designing these systems. Spray-annular

flow pattern occurs at higher qualities, and is of interest in most

two-phase systems. Bergles and Roos [15] conducted experiments

in recirculating high-pressure steam loop, which reduced the

expenditure considerably. Film thickness was measured with an

electrical probe, and was found to gradually decrease to zero as the

CHF was approached. Measurements in rod-bundles indicated

wide variations in film thickness over tubes.

Professor Bergles extensively used many visualization

techniques to obtain a good physical picture of complex

phenomena. Fiori and Bergles [25] developed a series of films to

study burnout in subcooled flow boiling.

Utilizing the experimental data and the photographic

information of the CHF phenomenon, Fiori and Bergles [33]

proposed a model in which stable dry spots are formed underneath

bubbles, and these spots can no longer be quenched at higher heat

fluxes, leading to vapor patches covering the heater surface. They

presented a comprehensive discussion on possible mechanisms

leading to CHF based on the information from Fastax (1200 frames

per second) camera and microflash photos.

Bergles [60, 72] surveyed the available literature and provided a

comprehensive coverage on the description of the burnout

phenomenon in pool boiling with different heater configurations,

and different CHF augmentation techniques. This paper presents

useful summary, and more importantly, future directions for

researchers. Similar reports were presented by Bergles [73] for the

low quality forced convection systems, and by Bergles [100] for

the high qualtiy forced convection systems. These comprehensive

surveys provide a clear picture of the parametric trends and effects

of important system variables on CHF. For the pool boiling

systems, Park and Bergles [195] collected 2237 data points for

CHF and fitted polynomial curve fits to provide engineering

equations for system designers.

In a shell and tube evaporator, the tube length covered by

baffles may be considered to be under pool boiling conditions.

Since the liquid supply is restricted, the burnout condition could be

initiated at this location. Jensen et al. [70] studied the dryout in

pool boiling under restricted annular geometries and found that the

dryout condition occurred at lower clearances and larger widths of

baffle coverage. However, the pool boiling curve shifted to the left,

indicating a more efficient heat transfer under the restriction.

Jensen et al. [70] attributed this increase to the thin film

evaporation in the clearance space.

CHF remains a major concern in high heat flux systems.

Vandervort et al. [241] conducted an experimental study in forced

convection systems with water in stainless steel tubes having

diameters ranging from 0.3 to 2.7 mm. Mass fluxes ranged from

5,000 to 40,000 kg/m2-s, and subcoolings ranged from 40 to 135

C. In some preliminary tests, a maximum heat flux of around 108

W/m2 was achieved. The CHF was shown to increase with both

velocity and subcooling. Small diameter tubes provided a higher

CHF. More detailed data are presented by Vandervort et al. [280].

In cooling of electronic chips with pool boiling liquid, the heater

thickness affects the CHF. Carvalho and Bergles [259] studied this

effect, and found that none of the conventional parameters such as

wall capacitance, thermal conductivity, or thermal diffusivity were

able to correlate the CHF data well. Carvalho and Bergles [259]

verified the new parameter conpacitance , which consists of the

heater thickness, and heater material thermal properties. Although

a considerable data spread is still observed, this work represents a

major step in formulating CHF for thin heater geometries. Using

the same parameters, Golobic and Bergles [260] proposed a new

correlation which correlated their own experimental data for strips

cooled on both sides with an average absolute deviation of less than

10 percent.

The mechanism of saturated pool boiling CHF was discussed

by Bergles [257]. The two competing theories, hydrodynamic

stability theory and microlayer dryout interpretation were

discussed. Knowledge of the flow pattern near CHF was deemed

necessary to clarify the situation for flat heaters, which forms the

basis for other geometries as well.

2.5.8 CHF in Helically Coiled Tubes. Helically coiled tubes

are used in industries for single-phase, evaporating and condensing

flows, and many other applications. At the system start up, the

subcooled boiling conditions sometime lead to the CHF condition,

which is not well studied in the literature. Jensen and Bergles [107,

126] conducted experiments to obtain CHF data with R-113 in

0.762 mm diameter tubes. The data was correlated and it was

found that an additional parameter consisting of non-

dimensionalized radial acceleration was able to account for CHF in

helically coiled tubes. The CHF in these tubes was lower than the

straight tubes. Undesirable upstream dryout was found to occur if

the coil was operated under low subcooling or low quality near the

inlet, and in the high quality region near the exit.

Jensen and Bergles [131] studied an interesting problem of

practical importance in solar energy applications. A helically

coiled tube in this application experiences a higher heat flux on the